FIELD OF THE INVENTION
The invention relates to a method for operating a fuel cell system, and a fuel cell system.
It is known that, during the electrolysis of water, the water molecules are broken down by electric currents into hydrogen and oxygen. In a fuel cell, this process is reversed. During the electrochemical combination of water and hydrogen to form water, electric current is produced with a high efficiency and, if pure hydrogen is used as the combustion gas, the electrochemical combination is effected without the emission of pollutants or carbon dioxide. Even with technical combustion gases, such as natural gas or coal gas, and using air or air enriched with O.sub.2 instead of pure oxygen, a fuel cell produces a considerably smaller amount of pollutants and less CO.sub.2 than other energy producing devices which operate with fossil energy sources. The technical implementation of the principle of the fuel cell has led to widely differing solutions, with different types of electrolytes and with operating temperatures between 80.degree. C. and 1000.degree. C.
A fuel cell block, which is also called a "stack" in the technical literature, is, as a rule, composed of a number of fuel cells stacked on top of each other.
The moistening and compression of the process gases before they enter the fuel cell block is problematic, since the vaporization enthalpy for moistening must be provided. Apparatuses having a membrane moistening or water injection after compression, are for example known from German Patent DE 43 18 818.
In addition, International Application WO 97/10619 discloses a method for operating a fuel cell system having at least one fuel cell block, in which at least one operating agent/process gas for the fuel cell block is compressed by means of a liquid ring compressor. After compression, water is separated from the operating agent and is fed via a product water container to the cooling circuit of the fuel cell system.
A disadvantage of the methods known from the prior art is that a complex structure of the fuel cell system is required for performing various method steps, such as for example separating water from a process gas for the fuel cell block, and providing the cooling water for the fuel cell block and an operating liquid for the liquid ring compressor. In other words, a complex structure means that individual components have to be provided in the fuel cell system for the various method steps which results in a high financial outlay.
Furthermore, a relatively large number of components in the fuel cell system also increases the requirements for controlling and regulating the individual components.